Unwanted crystalline defects and impurities can be introduced during crystal growth or subsequent wafer fabrication processes. These defect and impurities can degrade device characteristics and overall yield. Gettering has been described as a process for moving contaminants and/or defects in a semiconductor into its bulk and away from its top surface to create a denuded zone cleared from contaminants and/or defects. Preferably, devices are built in the denuded zone.
Historically, extrinsic backside gettering was used to getter silicon wafers. Various extrinsic backside gettering processes involve damaging the backside of the wafer mechanically or by implanting argon, germanium, hydrogen or other implants, or providing a gettering layer on the backside of the wafer using a phophorosilicate glass or oxide backside layer, a polysilicon backside layer, and a silicon germanium (SiGe) backside epitaxial layer. Subsequently, “intrinsic” gettering was developed, which employed oxygen precipitation and “bulk microdefects” precipitated into the bulk of the wafer after the surface was “denuded” of oxygen. The precipitation process, the gettering effects, and the electrical characterization of defects and gettering silicon wafers have been investigated. Recently, intrinsic gettering modifications have been developed, including neutron irradiation, high boron doping, nitrogen doping, and the use of magnetic fields during crystal growth.
These gettering processes depend on the diffusion of unwanted impurities over significant distances to the gettering sites. However, modern low temperature processes have small thermal budgets, and do not afford an opportunity for significant diffusion of dopants and/or unwanted impurities. Thus, it is desirable to reduce the distance between the gettering sites and the device area. It has been previously proposed to implant various impurities in proximity to the device areas, to co-implant oxygen and silicon to form a gettering layer in close proximity to the device area, to implant helium to form cavities close to the device areas which getter impurities, and to getter material in trench isolation areas in close proximity to the device areas.
However, silicon on insulator (SOI) wafers and technology pose problems for backside extrinsic gettering and intrinsic gettering because of the intervening oxide layer between the active device silicon layer and the substrate. A number of techniques have been proposed to getter silicon on insulator layers. These proposed techniques include: a front-side polysilicon layer; a phosphorous doped oxide layer or phosphorous doped silicon layer in the SOI structure; microcavities formed by implantation; scribe lines; materials in or stress caused by trenches in the SOI structure; damage introduced by hydrogen implants; phosphorous doped microcrystalline silicon layers implant damaged regions in close proximity to device areas; and a carrier substrate with nitrogen and carbon.